The invention relates to an apparatus and method for laser shock peening. In order to strengthen or otherwise change the mechanical properties of certain materials, such as metals, metal alloys, and heterogeneous structures thereof, peening has been employed. One type of peening involves shot peening, wherein small balls are fired against a surface of a target workpiece to create plastic deformation and corresponding residual stress in the target workpiece. The residual compressive stress improves the useful fatigue strength and operational life of the workpiece.
It is also known to use laser shock peening in place of conventional shot peening. A laser, typically a high-energy solid state laser, is operated in pulse mode and pulses are directed against a workpiece surface. The pulses generate mechanical shock waves, which produce compressive residual stresses that are capable of enhancing the fatigue strength and fatigue life in metals or metal alloy parts or structures.
The function of these compressive residual stresses (i.e., from conventional shot peening or laser shock peening) is to stop crack propagation by the establishment of additional forces that must be overcome by tensile forces under normal or abnormal mechanical operational conditions. One exemplary application of laser shock peening is in the manufacture of fan blades for turbine devices.
In laser shock peening, the workpiece to be treated is typically covered with a sacrificial light absorbing coating, which is generally an ablative material (e.g., black paint or tape). The coating is covered by a thin layer of water. The laser is positioned so as to direct pulsed laser energy to the coated workpiece. Absorbed laser energy vaporizes (ablates) the coating in a small explosion that is inertially confined by the water to develop an instantaneous pressure pulse or high pressure mechanical shock wave that is directed into the workpiece. The shock wave causes plastic deformation of the workpiece surface and corresponding residual compressive stress within the underlying microstructure of the workpiece.
While the aforementioned apparatus and process benefits certain mechanical properties, unwanted side effects may occur. For example, comparisons of the microstructure of a workpiece before irradiation and after irradiation according to the process of laser peening have demonstrated microstructural faults at the mid-plane between the surfaces of the workpiece. One microstructural fault typically appears to be in the form of dislocations (ragged cracks or tears) in the microstructure of the workpiece. These microstructural defects are generally undesirable as they may reduce the fatigue strength and impact resistance of the workpiece in its intended application.
A method for laser shock peening a surface is disclosed. A laser beam pulse from a laser apparatus is produced. The laser pulse has a cross-section taken perpendicular to the laser beam, a fluence profile across the cross-section. The pulse is directed against an area on said surface. The fluence profile is controlled such that process induced defects are minimized.